This invention relates to a turbine blade or turbine vane formed of a metallic nonfoam region and a ceramic foam region.
The property requirements of a turbine blade or turbine vane vary greatly according to location within the article. For example, the attachment (dovetail) must be strong and fatigue resistant at intermediate temperatures, the root region of the airfoil must be strong, fatigue resistant, and resistant to environmental damage at higher temperatures, and the tip region of the airfoil must retain a form factor and have excellent resistance to environmental damage at the highest temperatures. Different parts of a single region may require different properties, as for example the pressure side and the suction side of the airfoil. There is a large incentive to raise the combustion gas temperature of the engine. However, there is also a large incentive to decrease the weight of the turbine blades as much as possible, because a reduction in turbine blade weight leads to reductions in disk weight, shaft weight, bearing weight, and support weight that in turn increase the weight efficiency of the engine.
In most cases, the different property requirements are met with a single material of construction that may not be optimal for any one location but instead achieves a good balance of properties for all of the locations. The currently preferred material of construction for most turbine blade and turbine vane applications is nickel-base superalloys, which may be coated to protect against environmental damage at the highest temperatures.
Composite materials have been developed for use at room temperature and mildly elevated temperatures. Such composite materials include the familiar fiber-reinforced organic matrix composites such as graphite fiber-epoxy composites. Structures made of such materials may have their properties tailored according to the location within the article, by changing the direction of the fibers, the volume fraction of the fibers, the type of fibers, and the like.
There have been attempts to apply these principles of composite construction to high-temperature applications such as turbine blades and vanes. Research studies have been underway for many years to apply composite-construction principles to high-temperature components such as turbine blades. These efforts have focused on superalloys that are reinforced by particles, fibers, or whiskers of ceramic materials. Although there have been some advancements, these efforts have not been successful in the sense that there are no such composite articles in regular service today. Gas turbine blades are typically made of nickel-base superalloys that may be made hollow to reduce weight and to allow cooling air to be conveyed through the interior of the blades. The use of a composite construction would offer the promise of reducing weight while maintaining performance, but no operable approach has been proposed as yet.
There is, accordingly, a need for an improved approach to turbine blades and vanes that must operate at elevated temperatures, must have property requirements that vary substantially at different locations of the article, and must be as light in weight as possible. The present invention fulfills this need, and further provides related advantages.
The present invention provides a composite construction that is applicable to turbine blades and turbine vanes which operate at high temperatures. The structure utilizes a combination of metallic regions and ceramic foam regions to tailor the properties as required for excellent mechanical properties and for low weight. The approach of the invention allows the designer of the article to determine the required properties for the various locations, and then the article is manufactured with different materials optimized for each location.
An article of manufacture comprises an article selected from the group consisting of a turbine blade and a turbine vane. The article further comprises a metallic nonfoam region, and a ceramic foam region joined to the metallic nonfoam region. The ceramic foam region comprises an open-cell solid foam made of ceramic cell walls having intracellular volume therebetween. The ceramic cell walls are preferably alumina. The intracellular volume may be empty porosity or an operable intracellular metal such as an intracellular nickel-base superalloy. The ceramic foam region may even be varied within itself, to have a first ceramic foam subregion having an intracellular volume that is empty porosity, and a second ceramic foam subregion having an intracellular volume comprising the intracellular metal. The metallic nonfoam region may be any operable metal, such as a primary nickel-base superalloy. The nonfoam region and the ceramic foam region are joined by any operable approach, such as a weld joint, a diffusional joint, or a casting joint.
In one approach, a method is provided for preparing an article selected from the group consisting of a turbine blade and a turbine vane. The method comprises the steps of preparing an airfoil region by the steps of providing a piece of a sacrificial ceramic having the shape of the airfoil region, and contacting the piece of the sacrificial ceramic with a reactive metal which reacts with the sacrificial ceramic to form an oxidized ceramic of the reactive metal and a reduced form of the ceramic. The resulting structure comprises a ceramic foam of the oxidized ceramic compound of the reactive metal with ceramic cell walls and an intracellular volume between the ceramic cell walls, the intracellular volume comprising a reaction-product metal. The reaction-product metal may be removed to create empty porosity, replaced with a replacement metal, or left unchanged. The method further includes joining the airfoil region to an attachment region by any operable approach.
The present approach provides a great deal of flexibility in precisely tailoring the structure and properties of a turbine blade or turbine vane. These structures have in common an airfoil that is joined to an attachment structure. The ceramic foam material used in the airfoil is lighter in weight than a comparable superalloy, and the weight may be reduced even further by removing the reaction-product metal from the intracellular volume where mechanical property requirements are minimal and the material functions largely to define a form. Where the mechanical property requirements are higher, the reaction-product metal may be replaced with the intracellular nickel-base superalloy to produce a ceramic foam whose intracellular volume is filled with the superalloy.
The joining of the ceramic foam regions and the nonfoam regions is accomplished by any operable approach. In one technique, the regions are each fabricated separately and then joined by welding such as electrical resistance welding, solid-state diffusional joining, liquid-phase joining that may be possible in some cases, or brazing with a brazing metal. In another technique, the ceramic foam region is fabricated, and the metallic nonfoam region is cast around it.
The result is a turbine blade or vane that has the metallic nonfoam region where required for strength and ductility, typically in the attachment, and the ceramic foam region that has a high-temperature shape-retention capability but is not as strong and ductile as the metallic nonfoam region. The metallic nonfoam material is typically used to form the attachment, and the ceramic foam material is typically used to form some or all of the airfoil. Other features and advantages of the present invention will be apparent from the following more detailed description of the preferred embodiment, taken in conjunction with the accompanying drawings, which illustrate, by way of example, the principles of the invention. The scope of the invention is not, however, limited to this preferred embodiment.